Fuel System:
* Fuel Filter: A clogged fuel filter restricts fuel flow, leading to hesitation and sputtering, especially under load. This is one of the cheapest and easiest things to check first.
* Fuel Pump: A weak or failing fuel pump might not deliver enough fuel pressure, causing similar symptoms. Listen for the pump's whirring sound when you turn the key to the "on" position (before cranking). A weak or failing pump may only be audible faintly, or not at all. A fuel pressure gauge test is definitive.
* Fuel Injectors: Clogged or faulty fuel injectors can cause a misfire in one or more cylinders, resulting in sputtering and hesitation. A fuel injector cleaner might temporarily help, but replacement is often necessary.
* Fuel Pressure Regulator: A malfunctioning regulator could cause either too much or too little fuel pressure, leading to poor performance.
* EGR Valve: A stuck-open EGR valve can lean out the fuel mixture causing rough running.
Ignition System:
* Distributor Cap and Rotor: Worn or cracked distributor cap and rotor can cause poor spark delivery to the spark plugs, resulting in misfires.
* Spark Plugs and Wires: Worn, fouled, or damaged spark plugs and wires can prevent proper ignition. These should be inspected visually and replaced as needed. Worn wires can cause intermittent misfires under load.
* Ignition Coil: A failing ignition coil can cause weak or intermittent spark, leading to rough running.
* Ignition Module (Electronic Ignition Control Module): This controls the spark. Failure can cause all sorts of drivability issues.
Air Intake System:
* Air Filter: A severely clogged air filter restricts airflow, leading to a lean fuel mixture and poor performance.
* Vacuum Leaks: Leaks in the vacuum lines can disrupt the proper operation of various engine systems, causing erratic running. Listen for hissing sounds around vacuum lines.
Other Potential Issues:
* Throttle Position Sensor (TPS): A faulty TPS can provide inaccurate readings to the engine control unit (ECU), leading to poor fuel delivery.
* Mass Airflow Sensor (MAF) Sensor: A dirty or failing MAF sensor provides inaccurate readings of the air intake, causing the ECU to inject the wrong fuel mixture.
* Oxygen Sensor (O2 Sensor): A faulty O2 sensor can cause a rich or lean condition, affecting performance and fuel economy. Often throws codes which can be read with an OBDI reader.
* Catalytic Converter: A clogged catalytic converter can restrict exhaust flow, leading to performance issues. While less likely to cause sputters and hesitations as a primary symptom, it's possible.
Troubleshooting Steps:
1. Check the easy stuff first: Air filter, fuel filter, spark plugs, and wires.
2. Listen for unusual noises: Hissing (vacuum leak), whirring (fuel pump).
3. Check for error codes: If your vehicle has a check engine light, get the codes read using an OBDI scanner specific to that year. Some auto parts stores will do this for free.
4. Visual inspection: Look for any obvious damage or loose connections.
5. Systematic approach: Address the most likely culprits one by one, testing and replacing components as needed.
Important Note: Without a proper diagnostic, it's difficult to pinpoint the exact cause. A mechanic with experience on older vehicles and access to diagnostic tools would be the best bet for a thorough diagnosis and repair. Trying to guess can lead to unnecessary parts replacement and expense.
